Twinkle twinkle, little star, we can’t see you where you are

“I see nothing.”“What you do not see is a white dwarf with such low temperature that it’s solidified.”

Astronomers often have to infer the presence and properties of objects they can’t see directly. Sometimes, as with black holes, we have a lot of data, thanks to matter swirling around and falling in. Sometimes, however, only the gentle tug of gravity reveals a hidden object. That’s the case for a companion body to a known pulsar, but in this case the fact that astronomers can’t see anything tells them a lot about it. In particular, it matches the description of a white dwarf with such low temperature that it has solidified — almost, but not quite, like a diamond in the sky. My latest column in The Daily Beast tells the tale:

This binary system is right next door to Earth, in cosmic terms—it’s only about 900 light-years away. So the pulsar’s companion can’t be a star, since it would be visible in telescopes. It might be another pulsar, but it would have to be very low-mass for that type of object, and the orbit looks wrong compared with other two-pulsar binaries astronomers have studied. That leaves one other improbable but ultimately most likely culprit: a high-mass, low-temperature white dwarf. [Read more…]